In a conventional system for detecting lean and stoichiometric air fuel ratios without switching of circuits, in general, a specified voltage lower than an electromotive force shown on the rich air fuel ratio operation and higher than an impression voltage generating a threshold current value whereat a pump current takes a certain value, is impressed between two electrodes of a detecting element, the lean air fuel ratio is detected thereby as a continuous output from 0 to plus, a rich air fuel ratio is detected as a negative voltage corresponding to the difference between the electromotive force generated in a sensor and the aforesaid impression voltage, which is corresponding to a pump current in an inverse direction against the pump current in a lean air fuel ratio operation, and the stoichiometric air fuel ratio is detected by an output which changes sharply from 0 to minus, as is described, for instance, in FIGS. 4 and 7 of Japanese Patent Laid-Open No. 179351/1983 entitled "Oxygen Density Detecting Method", which was laid open in Japan on Oct. 20, 1983.
Since a signal of a rich region containing the stoichiometric air fuel ratio ( .lambda.=1) is a negative output, as shown in FIGS. 5 and 6 of the above-described prior art, the rich region needs to be inverted into a positive signal to be processed by a computer. Moreover, two different output values are present in .lambda.=1, in the course from the rich side to .lambda.=1, and in the course from the lean side to .lambda.=1, since a signal on the rich side and a signal on the lean side change sharply at a stoichiometric air fuel ratio point. Accordingly, this causes a drawback that the processing of the signal of .lambda.=1 is complicated. Furthermore, when the negative output on the side of rich is inverted to a positive output to obtain two positive outputs, whether the aforesaid output is the one on the rich side must be determined.
The above-stated problem is settled by a sensor processing circuit in which a potential ground V.sub.PG from 0 to a positive value is added to a reference potential of an O.sub.2 sensor over all the regions from the rich to the lean to obtain a continuous positive output voltage, as shown in FIG. 1 of Japanese Patent Laid-Open No. 180131/1986 entitled "Tri-state Air Fuel Ratio Sensor for Automobile", which was laid open in Japan on Aug. 12, 1986, for instance.
However, even this publicly-known example of Japanese Patent Laid-Open No. 180131/1986 can not get rid of a fault which makes it impossible to obtain a step signal at the point (.lambda.=1) of the stoichiometric air fuel ratio, as is the case with the present O.sub.2 sensor, while obtaining the extensive continuous output of the air fuel ratio.